Heterogeneous networks (HetNets) represent a substantial shift from classical cellular network deployment, as seen in homogeneous networks (HomoNets), towards an anarchic deployment, which may result in a high level of interference usually not experienced in HomoNets.
Generally, a HetNet may include a planned network of high power macro cells (which are typically deployed in a HomoNet) along with an unplanned network of low power cells (LPC), including femto cells, pico cells, relay nodes (RN), and so forth, interspersed within the high power macro cells. The LPCs may be added in or around areas with poor coverage and/or high user concentration to help improve overall communications system performance. In general, high power macro cells and LPCs may be referred to as communications controllers.
FIG. 1 illustrates a prior art HetNet 100. HetNet 100 includes a number of enhanced NodeBs (eNB), which are examples of high power macro cells, such as eNB 105, eNB 107, and eNB 109. eNBs may also be commonly referred to as controllers, base stations, NodeBs, and so forth. eNBs may be used to control communications to and from User Equipment (UE), with the eNBs assigning network resources for transmitting to the UEs and for receiving transmissions from the UEs. For example, eNB 105 may control UE 110, eNB 107 may control UE 112, and eNB 109 may control UE 114. UEs may also be commonly referred to as mobiles, terminals, subscribers, users, mobile stations, and so on.
HetNet 100 also includes a number of LPCs, such as LPC 120, LPC 122, and LPC 124. An LPC may control a UE by itself or it may operate in conjunction with other LPCs and/or eNBs to control a UE. As shown in FIG. 1, LPC 120 may control UE 112 in combination with eNB 107. Similarly, LPC 122 and LPC 124 along with eNB 109 control UE 114.
In the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 10 technical standards, certain channels and signals may be broadcast in predefined subframes by eNBs and/or LPCs (i.e., communications controllers). For example, a physical broadcast channel (PBCH) may be transmitted in subframe zero, a primary synchronization signal (PSS) may be broadcast in subframes zero and five for a frequency division duplexing (FDD) configuration and in subframes one and six for a time division duplexing (TDD) configuration, while a secondary synchronization signal (SSS) may be broadcast in subframes zero and five.
Even with the use of interference mitigation techniques, such as almost blank subframes (ABS), a common reference sequence (CRS) is transmitted on the first symbol of the subframe along with the PBCH, the PSS, and the SSS in appropriate subframes. Therefore, there is a considerable number of channels and signals being transmitted. In a HomoNet with a relatively small number of high power macro cells, e.g., eNBs, interference between the CRS, PBCH, PSS, SSS, and so on, is not a significant problem. However, in a HetNet with a potentially large number of LPCs as well as the high power macro cells, interference between the CRS, PBCH, PSS, SSS, and so on, broadcast by the communications controllers may be a significant problem. This problem is further exacerbated by the unplanned (or less planned) nature of LPC deployments. Since a LPC may be deployed close to an eNB and/or another LPC, a UE may suffer from high interference on the CRS, PBCH, PSS, SSS and so on.
Therefore, there is a need for a system and method for reducing interference.